1. Field of the Invention
The present invention relates to a zoom lens and, for instance, is suitable for image pickup optical systems of image pickup devices, such as a video camera, a digital camera, a TV camera, a monitoring camera and a silver halide film camera.
2. Description of the Related Art
An image pickup optical system adopted in an image pickup device is desirably a small zoom lens that has a wide angle of view and high optical performance over the entire zoom range to support wider imaging conditions. As a zoom lens which is small in its entire system and facilitates achieving a wide angle of view, a negative-lead type zoom lens has been known which includes a lens unit having a negative refractive power arranged nearest to an object. As a negative-lead type wide angle of view zoom lens, a four-unit zoom lens has been known which includes, in order from an object side to an image side, lens units having negative, positive, negative and positive refractive powers, and changes the intervals between adjacent lens units for zooming.
In this four-unit zoom lens, a first lens unit and a second lens unit constitute a lens group having a positive refractive power as a whole at a telephoto end zoom position, and the entire optical system can be a telephoto type. Accordingly, the focal length at the telephoto end can be increased. An inner focus type four-unit zoom lens has been known which has a focus lens unit arranged on the image side to achieve reduction in size and weight of the focus lens unit in the four-unit zoom lens. For instance, such lenses are disclosed in Japanese Patent Application Laid-Open No. 2006-58584, U.S. Pat. No. 7,777,967 and Japanese Patent Application Laid-Open No. 2001-343584.
Furthermore, a five-unit zoom lens has been known which includes first to fifth lens units having negative, positive, positive, negative and positive refractive powers, respectively, in order from the object side to the image side, and moves each lens unit for zooming. For instance, such a lens is disclosed in Japanese Patent Application Laid-Open No. H07-306362.
A negative-lead type zoom lens has a lens configuration asymmetry with respect to an aperture stop. Accordingly, correction of various aberrations is difficult. For instance, variation in aberration during focusing is large. It is thus difficult to achieve high optical performance.
In particular for high speed focusing, an inner focus type which uses a small and lightweight lens unit nearer to the image than a first lens unit to achieve focusing has a tendency to increase variation in aberration during focusing. To achieve a wide angle of view as well as reduction in size and high speed focusing a small and lightweight lens unit with respect to the four-unit zoom lenses and five-unit zoom lenses described above, it is important to appropriately configure the refractive powers and lens configurations of the lens units.
For instance, if the refractive powers of the focus lens unit and the final lens unit are not appropriately configured, it becomes difficult to achieve reduction in size of the entire system while achieving a wide angle of view and high optical performance over the entire zoom range and performing high speed focusing. In addition, large variation in imaging magnification during focusing unfavorably varies an area reflected in an image pickup range during focusing. In particular, while a moving image is being taken, a background object comes into and goes out of a frame during focusing in a process of focusing a subject. Since the variation can be easily observed, which is unfavorable.
Furthermore, particularly, a zoom lens having a wide angle of view typically has a higher depth of field than a telephoto lens has. Accordingly, a subject is not easily blurred, thereby unfavorably making the variation prominent. It is therefore important that the variation in imaging magnification during focusing is small.